Thérèse experiences a shift in her character. (More observing-humans "science" stuff here.) Instead of trying to rebel against the memory of Camille, she becomes strangely pious.
Not only does she praise her dead husband in front of Laurent, but she also starts begging Mme Raquin for forgiveness.
Every day, Thérèse prays for redemption. She even deludes herself into believing that Mme Raquin has forgiven her.
Laurent is greatly disturbed by this change in Thérèse, and worries that her repentance will lead her to reveal everything to the police.
Because he's such a sweet guy. Basically the only things he cares about now are not getting caught and not getting caught.
Thérèse now tortures Laurent every evening by telling him how much she loved Camille, and their rows end with Laurent beating her.
But Thérèse derives a strange sense of pleasure from being beaten, thinking that it provides a cure for the misery of her life. She deserves punishment, and she is punishment. (Scary thinking, for sure.)
Laurent also finds relief in striking Thérèse, and the couple become more and more violent toward each other.